The invention relates to thermal overload protection for protecting electrical devices, and particularly electric motors, from overheating.
Electric motors are utilized in several applications for driving various moving parts. An electric motor often has an associated control unit for adjusting and monitoring the operation of the electric motor, the speed of rotation, for example.
An electric motor may temporarily operate also overloaded, but if it becomes overheated as the loading continues, this may result in damage to the motor. Damage to the isolation of the stator coiling caused by overheating is the most critical.
Various solutions are known for protecting an electric motor against thermal overload. One known solution is based on 1 . . . 3-phase measurement of the motor current and on modelling the heating of the motor by using an RC equivalent circuit. The oldest and most common technical implementation is a bimetallic relay (thermal relay) coupled directly or via a current transformer to the main circuit.
A known solution is a thermal safety switch arranged inside or in connection with the motor, the switch tripping after a given temperature limit and interrupting the current flow through the electric motor. A more advanced version is an electronic unit that measures the temperature of the electric motor with temperature sensors and triggers a shut-off of the motor. This alternative manner is directly based on temperature detection with various sensors. The problem is the difficulty of placing the sensors correctly. Such a protection reacts relatively slowly.
In numerical protection, data is processed in a numeric format, i.e. digitally. Analogical measurement data are converted with an A/D converter into digital. The actual measurement and protection functions are implemented by means of a microprocessor. The thermal overload protection measures the root mean square (rms) values of the phase currents (load currents) of a motor or another object to be protected (e.g. a cable or a transformer), and calculates the temperature-dependent operating time. This thermal operating time may be accordant with standard IEC 60255-8:
  t  =      τ    ⁢                  ⁢    ln    ⁢                            I          2                -                  I          p          2                                      I          2                -                  I          b          2                                    wherein        t=operating time        τ=time constant        Ip=load current before overload        I=load current        Ib=operating current (maximum allowed continuous current)        
The thermal time constant τ is determined as the time required of the object to be protected to reach a temperature θ, which is a given portion (e.g. 63%) of a steady-state temperature θs, when the object to be protected is supplied with constant current. The operating current Ip is the highest allowed continuous current, which also corresponds to the highest allowed temperature, i.e. the steady-state temperature θs. This highest allowed temperature is the trip level. Alternatively, the relative value of the thermal load on the object to be protected relative to a full (100%) thermal load can be calculated from the phase currents. The trip occurs when the relative thermal load reaches a 100% value.
Numeric thermal protection is thus associated with heavy calculation requiring an efficient processor and fast and expensive peripheral circuits, such as memories. Prior art solutions have employed an efficient processor having also an in-built mathematics processor, a floating point unit (FPU) or a corresponding unit for performing real-time calculation within a determined time. An efficient processor having library functions emulating a floating-point number unit has also been used. Implementations also exist wherein the algorithm is implemented with ASIC circuits, whereby they cannot be reprogrammed afterwards. Consequently, changes cannot be made to such a single-purpose circuit, but a new circuit is always required if the operation is to be changed. Implementations also exist wherein the current is measured/calculated, the warming-up is calculated, measurements are repeated etc., in a sequence. Such an implementation does not ensure fully real-time protection (no continuous measurement), but enables the use of a less efficient processor.